The present invention is directed to hydraulic control systems, and pertains more particularly to an improvement in control systems such as disclosed in Ser. No. 583,366 entitled Part Throttle Control-Pump Override, now U.S. Pat. No. 3,963,378, assigned to the assignee hereof and having a plurality of variable displacement pumps and means for automatically adjusting the displacement of the pumps to engine output.
Many hydraulically acutated implements, such as a hydraulic excavator, normally employ a plurality of variable displacement pumps for supplying the high-pressure hydraulic fluid necessary to power the many motors employed in driving the vehicle and in manipulating the excavator bucket and linkage. Reference is made to U.S. Pat. No. 3,841,795 issued Oct. 15, 1974 to Ferre et al., assigned to the assignee hereof, for background material pertinent to the subject invention. The above patent was developed to overcome certain problems of the prior art and functions very satisfactorily so long as a single predetermined engine speed or engine normal operating speed is selected and maintained.
Earth-moving machines, such as the hydraulic excavator, are normally designed to have a predetermined normal operating speed of the engine during normal operation of the machine. With such machines, the engine throtte is normally set at a predetermined position when the machine is in normal operation.
Frequently, it is desirable to operate the vehicle at a lower or higher engine speed. This becomes a problem when the hydraulic system is equipped with a control system such as that of U.S. Pat. No. 3,841,795. With such a system, as soon as the engine speed is reduced, the differential pressure across the underspeed valve orifices would signal the pumps of the system to stroke to a lower displacement as they would respond as though the engine was running slower due to lugging of the engine due to overload. Thus, one would be unable with that system to operate the vehicle with the engine at a lower speed and at the same time obtain full pump flow from the pumps.
The control system disclosed in the aforementioned copending application Ser. No. 583,366 provides for adjusting the control system to respond to another speed other than the normal operating speed of the system. However, that too has limited flexibility in that only a single other speed is available.